HELP!!! Tenant(s) keeps leaving front communal door open

[Brandon Styles]

Hello BP'ers! 

I have a problem.  I have a tenant in my 4 unit apartment building that's leaving the front communal door open. Whoever is doing it is leaving the front door mat in between the front door. 

What can I do to stop this from happening? I want to leave a note/notice in the hallway and give each tenant a letter addressing the issue but I don't know what/how to say it. Also what else can/should I do besides writing a notice? Should I put up a security camera to show that I'm serious? Should I put some sort of alarm on the front door that goes off after x amount of minutes? Any and all advice is greatly appreciated!